F-14 calculated magnetic variation issues

[LynxOfTheSky]

Recently i decided to invest more time into RIO skills, starting with Persian Gulf 1992-3 event. And i encountered some effects, that i fail to understand, and i hope that you (or the developer of corresponding systems) can shed some light on them.

So we spawned cold on forestal carrier deck, year 2016. I used RF datalink CV alignment, all went normal. Magnetic variation for the starting location was +1.7 (as shown in kneeboard), vC (computed/calculated magnetic variation) was about -5.4 due to carrier being huge piece of metal. Everything seemed normal here.

Strange things started happening after take-off. As we go away from the carrier, vC should auto-correct itself (not immediately, but not too slow either, as far as i understand) to +-5 of the correct magnetic variation, but it was stuck around -3.4. After some flight and maneuvers it jumped up to +12, slowly correcting to something about +2.6.
Similar behavior continued for the whole event (we respawned for 4 or 5 times). There is a video of the event from my POV, but it's too long, still might be helpful: https://youtu.be/BFaRZhipFY8

While in flight, i started digging the manual, to find out, what is happening, finding out a part about vC and vM (page 272). Compasses correlated, so, as far as I thought, that was an IMU issue. But that seemed unlikely as well, cause we were probably 10-15 minutes in flight, and also datalink contact matched radar ones perfectly for the whole flight (until 12G maneuver by over-evasive pilot  ), which means INS was fine.

One advice that i got from Karon, creator of flyandwire, was pilot to hold HDG button, to synchronize DG with MAD. But that raised new questions, about how this DG + MAD system works.

So, my questions are:
1) what can be the cause of vC not self-correcting after take-off from carrier?
2) is it normal that vC jumped all upto +12 from initial -3.4 after a maneuver?
3) how bad in normal navigation (not TACAN fix) that error in vC is?
4) how does DG stabilize MAD? Is MAD value change just smoothed by DG? or both their values are taken into consideration and somehow computed?
5) just a theory, could DG possibly have error (due to carrier start), which resulted in wrong vC, but because MAD is a primary source for compass, reading from it were correlating with backup compass?

[sLYFa]

1) For the AHRS to sync to the magnetic heading, you need to be in level and unaccelerated flight. If you are maneuvering, the sync wont happen

2) That should not happen, however, I experienced the AHRS syncing to some wrong magnetic heading when doing steady turns. I suspect this is a bug but its hard to reproduce

3) Not bad at all. All the navigation calculations are done with INS provided true headings, the AHRS heading (which drives the BDHI and pilot heading tapes) is not used at all for navigation. TACAN radial indication should not be affected either since its does not rely on any heading reference. It does (wrongly) get affected by AHRS heading in DCS though (both BDHI needle and radial indicator)

4) DG (directional gyro) provides a reference to whatever it considers north. If you are in DG mode, you have to manually press and turn the HDG knob  until the heading tape/BDHI shows the correct heading (can be either true or mag, depends on what you need). In SYNC, the MAD in the vertical tail provides the reference for the AHRS gyro, i.e. the MAD knows where magnetic north is and tells the AHRS to align its reference to that. This is either done automatically if you are flying level or If you press the HDG knob in SYNC. Note that the latter does not guarantee an alignment of the AHRS to magnetic north, as the limitations on flight attitude remain and acceleration remain.

5) not really an answer but to clarify things. The AHRS directional gyro is what provides heading information to the pilot. The gyro is necessary since using the MAD directly would result in very unstable readouts during maneuvering (much like the whiskey compass). Theoretically, the gyro needs to "know" the direction of magnetic north only once (which it gets from the MAD during startup). But since the MAD-provided reference is wrong on the carrier, the AHRS gyro needs to get the correct reference somehow. This can happen either using the MAD during flight (again, level flight with as little pitch as possible is necessary for that), or manually by selecting DG mode and turning the HDG knob until the BDHI shows the correct heading (which you need to know beforehand, e.g. BRC-90° if you are parked on one of the starboard elevators).

Hope this helps

[LynxOfTheSky]

Thanks for your reply, but what is the source of vC then? Is it completely separate system from AHRS?

[sLYFa]

vC is the difference between AHRS supplied magnetic heading (which can be off) and INS true heading.

Edited January 27, 2022 by sLYFa

[LynxOfTheSky]

Yeah, found in manual, my bad, page 272: "Computed MAG VAR is constantly calculated in the AWG-9 by comparing the IMU’s true heading with the magnetic heading from AHRS.". Although, are you sure, that when vC is calculated, the vM is even used (added to the INS heading)?
Also another question, there are 4 digital sources of heading info to the pilot: HUD, VDI, HSD and BDHI, from where do they take heading information? Cause in manual (page 272) they suggest comparing headings on VDI (not from BDHI) with standby compass, so do all of them take heading from AHRS or only part of them?

Redacted:
Again, RTFM. Corresponding to it, BDHI feed information from AHRS (DG + MAD, or one of them, depending on selection), everything else shows you information based oin true heading and vC/vM, depending on situation (vC in most cases, vM when your INS has failed, or when RIO manually switched to one of backup modes)

Edited January 27, 2022 by LynxOfTheSky

[sLYFa]

The VDI and HSD will always show AHRS heading, unless there is some setting to feed them INS TH I don't know of